Allow the API_EXTERNAL_URL used by the gotrue service to be set

[toBeOfUse]

Is your feature request related to a problem? Please describe.
All of the email verification links generated when using a Supabase instance created by the CLI start with http://127.0.0.1. I believe that this is because this CLI always sets the API_EXTERNAL_URL environment variable to http://127.0.0.1 when creating the supabase/gotrue container. This is a problem when creating a Supabase instance using the CLI, and then trying to use it from outside of the current host, where 127.0.0.1 no longer refers to the machine that the gotrue container is running on.

Describe the solution you'd like
Ideally, the hostname in the links that the gotrue service creates would not always be 127.0.0.1. I believe that if a configuration option to set the API_EXTERNAL_URL option existed, it would allow us to change the generated URLs to use a different address or domain.

Describe alternatives you've considered
If there is any alternative way to set the hostnames that are used in email verification URLs, that would also be great.